Comparison of two different kinds of seed sludge and characterization of microorganisms producing hydrogen and soluble metabolites from raw glycerol

Glycerol is a by-product generated in the biodiesel industry and can be converted by biological processes into products such as hydrogen, alcohols and acids. This study aimed at comparing two sources of inoculum in dark fermentation using raw glycerol and at identifying the microorganisms present by high-performance sequencing. Granular sludge obtained from a vegetable oil industry (VOI) and anaerobic sludge from a wine industry (WI) were evaluated in batch reactors with raw glycerol. The highest hydrogen yield was obtained for VOI (1.12 mol H₂ / mol glycerol_consumed). The WI was more efficient in producing soluble metabolites, with 6.2 g / L of 1,3 – propanediol, ethanol (2.5 g / L) and acids (122 mg / g glycerol_consumed). Both assays consumed more than 60% of the glycerol. A significant variation in microbial diversity was observed between the beginning and the end of the fermentation. The microbial community showed a predominance of Firmicutes and the Clostridiaceae and Peptostreptococcaceae families in different abundances among the inocula, which could lead to an increase in hydrogen production or a higher concentration of metabolites. Clostridium bifermentans was the only species isolated, showing a yield of H₂ (0.135 mol H₂ / mol glycerol_consumed) and the main metabolite produced was ethanol (670 mg / L). These results showed that two different kinds of seed sludge can lead to different metabolic profiles from the same substrate, highlighting the importance of choosing the inoculum and, consequently, that the knowledge regarding microorganisms can direct the process towards products of industrial interest and contribute to the generation of sustainable energy. Graphical abstract: [Figure not available: see fulltext.]